The present invention relates to a catalytic reduction of nitrogen oxides. More particularly, it relates to a catalytic reduction of nitrogen oxides in an exhaust gas containing nitrogen oxides such as nitrogen monoxide and nitrogen dioxide (hereinafter referring to as NO.sub.x) and sometimes sulfur oxides (hereinafter referring to as SO.sub.x) which is discharged from a boiler, an internal combustion engine, a coke oven, a sintering furnace, and other industrial sources.